Xerographic development

ABSTRACT

IMAGES ARE FORMED ON AN ARCUATE IMAGING SURFACE BEARING AN ELECTROSTATIC LATENT IMAGE BY TRANSPORTING THE IMAGING SURFACE IN A GENERALLY ASCENDING ARCUATE PATH IN A CONTACT ZONE WITH A BATH OF DEVELOPER MATERIAL SUPPORTED IN A CONCAVE CHAMBER ADJACENT TO THE LOWER FACE OF THE IMAGING SURFACE, THE CONTACT ZONE EXTENDING UPWARDLY FROM ABOUT THE LOWERMOST POINT OF THE ARCUATE PATH TO A POINT SUFFICIENT TO ALLOW FRICTIONAL CONTACT BETWEEN THE DEVELOPER AND IMAGING SURFACE ALONG THE CONTACT ZONE TO CIRCULATE THE DEVELOPER IN THE BATH.

Sept. 20, 1971 R. w. GUNDLACH 3,606,533

XEROGRAPHIC DEVELOPMENT Original Filed Feb. 21. 1966 INVENTOR. ROBERT w. GUNDLACH wk Q United States Patent 01 dice 3,606,533 XEROGRAPHIC DEVELOPMENT Robert W. Gundlach, Victor, N.Y., assignor to Xerox Corporation, Rochester, N.Y. Original application Feb. 21, 1966, Ser. No. 528,846, now Patent No. 3,503,776, dated Mar. 31, 1970. Divided and this application July 2, 1969, Ser. No. 870,834

Int. Cl. G03g 15/00 U.S. Cl. 355-3 5 Claims ABSTRACT OF THE DISCLOSURE This is a division of application Ser. No. 528,846, filed in the United States on Feb. 21, 1966 and now Patent No. 3,503,776.

This invention relates in general to xerography, and more specifically to a system of developing xerographic images.

In the art of Xerography, as originally disclosed by Carlson in US. Patent 2,297,691, and as further described by many related patents in the field, the xerographic plate containing a photoconductive insulating layer is first given a uniform electrostatic charge in order to sensitize its entire surface. The plate is then exposed to an image of activating electromagnetic radiation such as light, X-ray or the like which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind a latent electrostatic image in the nonilluminated areas. This latent electrostatic image is then developed and made visible by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. Where non-reusable photoconductive insulating material is used, the marking material or toner particles are directly fixed in place on the surface by any convenient means such as by heat fusing. Where a reusable photoconductive insulating material is used, the Visible image formed by the electroscopic marking particles is transferred to a second surface, such as a sheet of paper, and fixed in place thereon to form a permanent visible reproduction of the original image.

The most widely used method of development today is cascade development. In cascade development, a finely divided pigmented electroscopic powder or toner, and a larger carrier material which acts as a vehicle for the electroscopic powder is used to develop the latent electrostatic image. The carrier material is larger in size than the electroscopic powder with said carrier being triboelectrically charged with a polarity opposite that of the electroscopic powder. The electroscopic powder is attracted to and surrounds the larger particles of carrier material and when cascaded over a xerographic plate, drum, or the like, the charge in the image area of the plate which is controlled to have a greater attraction for the electroscopic powder particles than the carrier material, attracts the electroscopic powder to said image area to form an image of toner particles. This image may be permanently fixed in place or transferred to another suitable support means such as a paper sheet. US. Patents to Walkup et al., 2,573,881; Eichler, 2,965,868; Walkup, 2,987,660; and

3,606,533 Patented Sept. 20, 1971 Carlson, 2,990,278; all illustrate the well-known cascade method of development. In all of the conventional cascade systems disclosed by the above patents, it is essential that a developer bucket arrangement for carrier and toner mixing and circulation be included in the developing system. The need for a bucket conveyor adds greatly to the size and cost of a xerographic system. It is also well-known that during the cascade method of development the carrier beads and unused toner particles which drop by gravity to the bottom of the developer chamber, generate a cloud of toner particles which result in an undesirable high background deposition in non-image areas. In addition, conventional cascade systems cause high drum and carrier abrasion due to an inherent sand blast eifect as the carrier and toner particles cascade against the moving drum or plate. An additional requirement in present cascade development systems concerns the need for thorough mixing of toner and carrier particles.

There is, therefore, a need for a developing system which avoids the undesirability of high background, provides for thorough toner and carrier mixing, which extends drum and carrier life, and eliminates the need for developer handling buckets.

It is, therefore, an object of this invention to provide an improved system of developing xerographic images which overcomes the above noted disadvantages.

It is another object of this invention to provide a sys tem of xerographic development which reduces the abrasion efiect presently a problem in cascade development.

It is a further object of this invention to provide a xerographic developing system which yields cleaner background over conventional cascade developing techniques.

It is yet a further object of this invention to provide for a developing system having greater simplicity than conventional developing techniques.

The foregoing objects and others are accomplished in accordance with this invention by providing an apparatus for xerographic development wherein the xerographic drum, plate, or the like is merely turned or passed against a bath of developer. In this invention, as hereinafter illustrated by a xerographic drum, the drum after charging and exposing to form an electrostatic image, is pulled through a bath of developer consisting of any conventional carrier beads and toner particles.

Electroscopic toner and carrier compositions are wellknown to those skilled in the art. Among the patents describing such compositions are U.S. Patents 2,618,551 to Walkup, 2,618,552 to Wise, 2,638,415 to Walkup and Wise, 2,659,670 to Copley and 2,788,288 to Rheinfrank and Jones. Toners generally have an average particle diameter between 1 and 30 microns, while the carrier beads are larger and may range from about 250 to 700 microns in diameter. The carrier and developer particles, may for example, be contained in a C shaped developing chamber and developing carried out merely by pulling or rotating the xerographic drum, while in direct contact with the developer, through the developer chamber. It has been discovered that the developer bath becomes self-circulating as the xerographic drum is rotated in contact with the developer particles. It should be noted that no external agitation means much as mechanical or fluidizing means are necessary for circulation or mixing of the developer.

While the developer chamber is generally parallel to the moving xerographic drum, and therefore roughly C shaped, it should be understood that the outer shell of the developer chamber may be of any suitable shape which allows for the conditions necessary for development as set forth in the specification. A typical shape includes an -L shaped developer chamber formed by a vertical side section and a horizontal bottom or base. Similarly, even when in the C shape, the developer chamber may be modified, such as by making the top or upstream end of the chamber wider than the bottom or downstream end to allow for a more efiicient circulation of the developer bath.

In general, the carrier beads may be of any suitable size. Typical ranges are from 250 to 700 microns in diameter. Similarly, any suitable toner concentration may be employed, with concentrations ranging approximately from V2 to 2 percent by weight being suitable. The toner particles may be replenished by adding toner to the top of the developer chamber with the self-circulating developer bath automatically mixing the added toner.

The advantages of this improved xerographic development apparatus will become apparent upon consideration of the following disclosure of the invention; especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view of one form of apparatus for carrying out the novel method'set forth in the specification.

FIG. 2a is an enlarged schematic view of the developer chamber of FIG. 1.

FIG. 2b is a schematic view of the xerographic drum illustrating the areas of the drum surface which relate to the developer chamber of FIG. 1.

FIG. 3 is a schematic view of a second form of apparatus for carrying out the method set forth in the specification.

Referring to FIG. 1, reference character 1 designates a rotatable xerographic drum having an outer layer of photoconductive insulating material such as vitreous selenium. The surface of the drum is uniformly charged by conventional corona charging device 2, and exposed to a pattern of activating electromagnetic radiation at 3. The latent electrostatic image formed by 3 is developed by rotating drum 1 through a self-circulating developer chamber 4 containing a cascade developer 5 consisting of substantially spherical carrier beads and a suitable marking toner. As the drum 1 rotates in direct contact with the developer bath, the carrier beads and attached toner particles flow in chamber 4 in a counterclockwisemotion thus rendering the developing chamber self-circulating. Upon leaving the developer chamber, the latent electrostatic image is coated with toner particles. Fur brush 6 may be employed to dislodge unwanted carrier beads at the fringes of the image areas without adversely effecting the developed image. This is unnecessary for beads greater than about 500 microns diameter and density of 5 or more. The developed image is transferred at transfer station 7 to a moving paper web. The transferred image may be permanently fixed to the paper by any convenient means such as. heat fusing. The drum is then cleaned of any excess toner particles at cleaning station 8 at which time it has completed the entire developing and image transfer cycle.

FIG. 2a is an enlarged view of the self-circulating developer chamber wherein the rotation of xerographic drum 1 of FIG. 1 through chamber 4, containing developer 5, causes a counterclockwise self-circulation of the developer bath due to a combination of the chamber shape, and the friction between therotating xerographic drum and the carrier particles. The small arrows in the developer bath indicate the direction of flow of the developer material. It can be seen that when additional toner is added to the developer bath, uniform mixing of the toner and carrier particles is easily attained due to the self-circulating action of the developer bath.

As further illustrated in FIG. 2a, the developer chamber need not be uniformly spaced from the xerographic drum, but in fact optimum results are attained when the distance between the outside of the developer chamber and the drum is wider near the top where circulation of the developer bath is greatest. This is illustrated by distance 9 in FIG. 2. The are of the drum covered by the developer bath of the developer chamber need only be about of the total drum surface in order to operate satisfactorily. A segment of the chamber beginning substantially vertically below the drum does not appear to undergo any developer flow and acts merely as a seal during the developing step. This is illustrated by area 10- in FIG. 2a which functions more or less as a seal in that no substantial developer flow or circulation is observed in this area. It can be seen from FIG. 2b that only about 165 or less of the surface of the drum need be encompassed by the developer bath.

In FIG. 2b reference character 1 designates the xerographic drurn used in FIG. 1. Section X of the drum illustrates the area of the drum, when rotating in a clockwise manner, which would be included within the developing area of the developer chamber of this invention. Area Y illustrates the area of little or no developer circulation which may optionally be included within the developer chamber area. Area Z illustrates the area of the drum surface not in contact with the developer bath.

In FIG. 3, another embodiment of the invention is shown wherein two drums employing a flexible xerographic belt are used allowing full frame exposure. In this embodiment rotating drums 11 and 12 carrying a flexible xerographic belt 13 is charged to a uniform potential by corona charging device 14. The belt is then exposed to activating electromagnetic radiation at 15 and developed in self-circulating developer chamber 16 containing developer 17.

The toner image on the belt 13 is transferred to a transfer web' 18 and the image subsequently made permanent by heat fusing. The belt is then cleaned at brush 19 after the developing and image transfer cycle is completed.

The concept of this invention may be adapted to any type of carrier, toner, or photoconductive material. Carrier ranges in a size of from about 250 to 700 microns have been tested and found satisfactory. The carrier beads may be of any convenient material such as glass or metal, coated with plastic. Similarly, any, conventional toner described above may be used in amounts ranging from about /2 to 2 percent. In addition, this process is adaptable to any photoconductive surface such as vitreous selenium, organic or inorganic photoconductors imbedded in a non-photoconductive matrix, or organic or inorganic photoconductors imbedded in a non-photoconductive matrix, etc. Such photoconductors are illustrated in US. Pats. to Ullrich, 2,803,542; Bixby, 2,970,906; Middleton et al., 3,121,006 and 3,121,007; and Corrsin, 3,151,982.

The following examples further specifically define the present invention with respect to a method of developing xerographic images in a self-circulating developer chamber. The parts and percentages in the disclosure, examples, and claims are by weight unless otherwise indicated. The examples below are intended to illustrate the various preferred embodiments of carrying out a developing method in a self-circulating developer chamber.

EXAMPLE I A xerographic drum coated with a 50 micron layer of vitreous selenium is corona charged to a voltage of about 400 volts and exposed to activating electromag netic radiation to form a latent electrostatic image on its surface. The selenium drum is then rotated through a self-circulating developer chamber such as that illustrated in FIG. 1 containing 250 micron plastic glass beads and 1.2 percent of a pigmented polystyrene-butyl methacrylate blend developing toner. The drum is rotated at a speed of approximately 10 inches per second. After one rotation through the developer chamber the image on the drum is transferred to a sheet of paper and fixed by heat fusing. An excellent image is obtained by this process.

EXAMPLE II A xerographic drum coated with a 50 micron layer of vitreous selenium is charged to a voltage of approximately 500 volts and then exposed to light to form a latent electrostatic image. The drum is then rotated through a self-circulating developer chamber such as that illustrated in FIG. 1 containing plastic coated lead beads of about 700 microns diameter and 2 percent of a rosin modified phenol formaldehyde resin developing toner. The speed of rotation is approximately inches per second. The developer or toner image on the drum is transferred to a sheet of paper and made permanent by vapor fixing. A high quality image is obtained from this method.

EXAMPLE III A xerographic drum coated with a micron layer of vitreous selenium is corona charged to a voltage of approximately 600 volts and exposed to form a latent electrostatic image. The selenium coated drum is then rotated through a self-circulating developer chamber containing a bath of 700 microns plastic coated lead beads of a pigmented polystyrene-butyl methacrylate blend developing toner. The drum is rotated at a speed of about 10 inches per second. The developer image is transferred to a sheet of paper and permanently fixed by heat fusing.

In a 50,000 cycle test, no evidence of abrasion of the selenium surface was seen after 50,000 revolutions through a developer chamber of the type illustrated in FIG. 1 of the drawings. During this test, the developer also showed no evidence of abrasion.

The method of the present invention is usually carried out in a xerographic system which includes at least the three basic steps of charging, exposing, and developmg.

Important applications exist .in which xerographic recording is desired intermittently, or on a variable speed basic. Facsimile transmission of documents and computer output recording are examples in which images are presented and must be recorded at varying rates, or even in a start-stop operation. The present invention has been shown to be admirably suited to such operations. Because developer circulation is caused by rotation of the xerographic drum itself, the quality of the developed image seems remarkably unaffected by changes in the drum surface speeds up to at least 40 inches/sec. That is, the total interaction of developer with the xerographic surface is evidently independent of the drum surface speed.

Although specific components, proportion and procedures have been stated in the above descripiton of the preferred embodiments of the novel developing method, other suitable materials, as listed above, may be used with similar results. In addition, other materials and procedures may be employed to synergize, enhance or otherwise modify the novel method. For example, additional toner may be added in any convenient manner such as by hand or automatically to the top of the developer chamber. In addition, the coefficient of friction between the surface of the xerographic drum and the developer material in the developer chamber may be controlled by varying the types of developer and/or the surface of xerographic drum to yield optimum results not inconsistent with good image development. Although corona charging is used in the examples, it should be stated that any suitable method of attaining an electrostatic image by charging the drum would be included with the scope of this invention. Other typical techniques which could be employed to yield an electrostatic image include the use of a pin matrix as a print head and pin tubes. Induction charging and the use of a conductive rubber roller with a potential applied between the conductive core of the roller and the conductive backing of the photoconductor could also be used in place of corona charging.

Other modifications and ramifications of the present invention would appear to those skilled in the art upon the reading of the disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. An apparatus for forming images comprising an imaging member having an endless electrophotographic imaging surface adapted to travel along a circuitous path in a vertical plane, means for forming an electrostatic latent image on said imaging surface, means to transport said imaging surface bearing said electrostatic latent image in a generally ascending arcuate path, a chamber having an impervious concave bottom adjacent to and generally concentric to said arcuate path, said chamber being adapted to support a substantially dry developer comprising electroscopic marking particles and carrier beads in contact with said imaging surface along an arcuate contact zone, said contact zone extending upwardly from at least about the lowermost point of said arcuate path through less than about of are along said arcuate path, said contact zone having a length along said arcuate path sufficiently long to permit deposition of at least a portion of said toner particles to form a toner image on said imaging surface in conformance to said latent image and to permit said imaging surface to circulate said dry developer material in said bath by frictionally transporting at least a portion of said dry developer material along the upper extremity of said contact zone, said chamber being further adapted to provide substantially quiescent developer material along the lower extremity of said imaging surface to form a seal for said bath of developer material, means to transfer said toner image to a receiving member and means to clean any residual toner particles from said imaging surface prior to the formation of a subsequent electrostatic latent image on said imaging surface.

2. Apparatus according to claim 1 wherein said concave bottom is spaced from said imaging surface at a greater distance at the upper portion of said contact zone than at said lowermost point.

3. Apparatus according to claim 1 wherein said imaging member comprises a belt.

4. Apparatus according to claim 1 including means to dislodge carrier particles from said imaging surface prior to transfer of said toner image to a receiving member.

5. Apparatus according to claim 1 wherein said means for forming comprises means to uniformly electrostatically charge at least a portion of said imaging surface and means to expose the electrostatically charged portion of said imaging surface to a light-and-shadow image.

References Cited 3,411,932 1l/l968 Malone et al.

MORRIS KAPLAN, Primary Examiner US. Cl. X.R. 118-637 

